1. Field of the Invention
Aspects of the present invention relate to a fuel processor that includes a reformer to produce hydrogen for a fuel cell using a gaseous fuel or liquid fuel; a shift reactor to decrease the concentration of CO, which is a by-product produced by the reformer; and a preferential oxidation (PROX) reactor. More particularly, aspects of the present invention relate to a fuel processor having a movable reformer burner within the reformer, a method of operating the reformer burner, and a fuel cell system incorporating the movable reformer burner within the reformer.
2. Description of the Related Art
A fuel cell is an electrical generation system that transforms chemical energy directly into electrical energy through a chemical reaction between oxygen and hydrogen, which are removed from the hydrocarbon groups of such materials as ethanol or propanol, or from hydrocarbons such as natural gas or butane.
A fuel cell system includes a fuel cell stack and a fuel processor as main components, and a fuel tank and a fuel pump as auxiliary components. The fuel cell stack has a stacked structure containing a few to many unit cells, wherein each unit cell includes a membrane electrode assembly (MEA) and a separator.
A fuel processor produces hydrogen by reforming a fuel, and the produced hydrogen is supplied to a fuel cell stack. In the fuel cell stack, the hydrogen electrochemically reacts with oxygen to generate electrical energy and form water. The fuel processor reforms hydrocarbon chains or groups using a catalyst. If the starting fuel material contains sulfur, the catalyst can be easily poisoned and lose activity such that the efficiency of the process is compromised. Therefore, it is necessary to remove the sulfur compound from the fuel before the hydrocarbon groups of the fuel are reformed in the fuel processor. Accordingly, the fuel is processed in a desulfurizer before it is fed to the fuel processor for reformation and hydrogen production.
The reformation of the hydrocarbon chains produces hydrogen, carbon dioxide (CO2), and carbon monoxide (CO). Unfortunately, the CO acts as a catalyst poison to the catalytic layer of electrodes in the fuel cell stack. Therefore, the reformed fuel must be processed so as to remove CO before the fuel is supplied to the fuel cell stack. The content of the CO in the fuel supplying to the fuel cell stack is preferably reduced to less than 10 ppm.
In general, CO is removed according to Reaction 1 at both high and low temperatures described as a high temperature shift reaction and low temperature shift reaction, respectively.CO+H2O→CO2+H2  [Reaction 1]
The high temperature shift reaction is performed at a temperature of about 400 to 500° C., and the low temperature shift reaction is performed at a temperature of about 200 to 300° C. However, the CO concentration in the fuel remains approximately 5000 ppm although the fuel is processed with the shift reactions.
To reduce the concentration of CO to about 10 ppm level, a preferential oxidation (PROX) reaction, shown below in Reaction 2, and a methanation reaction, shown below in Reaction 3, are used.CO+½O2→CO2  [Reaction 2]CO+3H2→CH4+H2O  [Reaction 3]
FIG. 1 is a configuration of a conventional fuel cell system that includes a fuel processor.
Referring to FIG. 1, in a fuel cell system that uses a gaseous fuel, the gaseous fuel is simultaneously supplied to a reformer 40 and a reformer burner 30 from a gaseous fuel tank 10. The gaseous fuel that enters into the reformer 40, for example, a city gas, generally requires a desulfurizer 14 to remove sulfur from the fuel to protect the catalyst within the reformer 40. The gaseous fuel that has passed through the desulfurizer 14 must have a sulfur content of less than 10 ppb.
The reformer burner 30 heats the reformer 40 to maintain the reformer 40 at a reformer temperature of approximately 750° C.
A liquid pump 22 supplies water to the reformer 40 from a water tank 20. The water supplied to the reformer 40.by the liquid pump 22 is preheated by passing through both first and second heat exchangers 71 and 72. A combustion gas from the reformer burner 30 is exhausted to the atmosphere after passing through the first heat exchanger 71, while the hydrogen-rich fuel produced by the reformer 40 is passed through heat exchanger 72 to further heat the water before the water enters the reformer 40.
In the reformer 40, hydrogen, carbon dioxide, and carbon monoxide are generated. The shift reactor 60 decreases the CO concentration in the generated fuel to a predetermined level, for example, 5000 ppm or less. The concentration of CO in the fuel is further reduced to less than 10 ppm in the PROX reactor 65. With the CO concentration sufficiently small upon exit of the PROX reactor 65, the hydrogen-rich, CO deficient fuel is supplied to the fuel cell stack 50.
The reformer burner 30 heats the reformer 40 and, at the same time, preheats the water through first heat exchanger 71 and maintains the temperature of the shift reactor 60 at a predetermined set-point temperature, for example, of about 250° C. using a combustion gas. The reformer burner 30 heats an inner space of a combustion chamber (see FIG. 2), and transfers heat to the reformer attached on surfaces of the combustion chamber 90. Such heat transfer heat to the reformer 40 provides energy for the reformation of the hydrocarbon groups to hydrogen. Typically, the reformer burner 30 is fixed within the combustion chamber of the fuel processor.
When the amount of hydrogen supplied to the fuel cell stack 50 is increased, the reformer temperature of the reformer 40 is reduced since the load on the reformer 40 is increased. So, to maintain the predetermined set-point temperature of the reformer 40, the amount of fuel supplied to the reformer burner 30 may be increased. However, the increase in the amount of fuel delivered to the reformer burner 30 increases the size of a flame, which may damage the combustion chamber. Furthermore, the increase in the amount of fuel supplied to the reformer burner 30 may cause a decrease in combustion efficiency of the reformer burner.
When the amount of hydrogen supplied to the fuel cell stack 50 is decreased, the load on the reformer burner 30 is decreased as the fuel cell stack 50 does not require as much hydrogen to continue operating efficiently. Thus, the amount of fuel supplied to the reformer burner 30 should be decreased.
An optimal position of the reformer burner 30 in the combustion chamber is beneficial for the fuel processor to operate efficiently. If the reformer burner 30 is improperly positioned heat cannot be efficiently transmitted to the reformer catalyst thereby reducing the combustion efficiency of the reformer burner in the combustion chamber and decreasing the overall efficiency of the fuel cell system.